A concordans is a translation table that gives you a mapping of a former numbering system to a newer numbering system.
For example, a concordans may exist to show how old addresses translate to new addresses. I used such a table in my research into my great-grandmother Cornelia Francisca van den Heuvel to show that address C 453 in Breda was the same as Leuvenaarstraat 158. It is rare to find such translation tables for addresses, so I got lucky.
Concordansen are sometimes provided in finding aids, if record groups are reorganized and the records get new call numbers. Old literature references may give you a number that is no longer in use. The introduction of the current finding aid may refer you to a concordans that will tell you what the current call number is to look up the record.